1. Field of the Invention
The present invention relates to a system for monitoring an optical signal, and more particularly, to a system for monitoring a wavelength-multiplexed optical signal.
2. Background
Optical signals generated by typical dense wavelength multiplexers (DWDMs) are generally multi-channel wavelength-multiplexed signals with relatively close wavelength separations between adjacent channels. The existence of signal channels in a wavelength-multiplexed signal produced by a typical DWDM is typically detected by filtering the wavelength-multiplexed signal with either conventional narrowband tunable optical filters or conventional wideband tunable optical filters. Narrowband tunable filters may be used to provide a spectrum of a wavelength-multiplexed optical signal to obtain the signal-to-noise (S/N) ratio, gain, gain-tilt and other physical parameters. However, the bandwidths of conventional narrowband tunable filters are usually too narrow to allow proper selection and transmission of individual channels existing in a typical dense wavelength-multiplexed optical signal. Furthermore, narrowband tunable optical filters are usually expensive.
Conventional broadband tunable optical filters have also been used for channel selection and transmission of wavelength-multiplexed optical signals. While broadband tunable filters typically have bandwidths that are sufficiently wide to allow signal transmission, they usually do not have sufficiently narrow bandwidths to properly reject adjacent channels and noise when an individual channel in a typical dense wavelength-multiplexed optical signal is selected for transmission. However, broadband tunable optical filters are usually less expensive than narrowband tunable optical filters.
There is a need for a DWDM test and monitoring apparatus which is capable of effectively detecting the existence of signal channels in a dense wavelength-multiplexed optical signal while avoiding the disadvantages of direct detection of channels by either conventional narrowband tunable filters or conventional broadband tunable filters.
The present invention satisfies this need. In accordance with the present invention, a method for detecting channels in a wavelength-multiplexed optical signal comprises the steps of providing an initial estimate of channels to obtain a coarse spectral match, performing a coarse nonlinear deconvolutional algorithm on the wavelength-multiplexed optical signal, and finding peak power levels of the signal in the initially estimated channels. The signal channels are selected from the initially estimated channels based upon the peak power levels. In an embodiment, power levels in the signal channels of the wavelength-multiplexed optical signal are determined with refined accuracy by performing a fine nonlinear deconvolutional algorithm. In a further embodiment, the coarse and fine nonlinear deconvolutional algorithms are performed by using Jansson""s nonlinear iterative method. The method is applicable to the testing of wavelength-multiplexed optical signals generated by broadband dense wavelength division multiplexers (DWDMs) and other optical communications devices.
Advantageously, the method according to the present invention allows signal channels existing in a wavelength-multiplexed optical signal to be determined with a small number of computational iterations. Furthermore, the power levels within the signal channels can be determined with a fine resolution in an embodiment according to the present invention. Furthermore, a relatively inexpensive broadband scanning filter can be used to scan the passband across the range of wavelengths covering the initially estimated channels, thereby saving the cost of hardware.